The present invention relates to a capacitive system comprising a dielectric sheet having a first electrode (touch pad) on one side thereof and on the other side thereof in capacitive relation with the first electrode, second and third electrodes which are mutually spaced.
Such capacitive systems are useful inter alia in touch control switching systems. In such switching systems, an AC signal is applied to the second or third electrode and an output signal circuit is connected to the other of them, so that when the first electrode is touched with a finger, the resulting change in the capacitive coupling between the second and third electrodes via the first gives rise to an output signal which may be used to effect the desired switching. These switching systems may be used in many ways, for example as computer inputs, in telephone "touch dialling" systems, in lift or elevator control systems and in control systems for domestic appliances such as cookers. For reliable operation of the control systems there is a general requirement that the modification of capacitance of the touch control system between the untouched and the touched condition should be as high as possible to facilitate detection of the modification by the detection circuitry. It is also desirable that the capacitance of the system should also be as high as possible. The reason for this is that the signal applied by a pulse generator to the electrode should keep the highest level possible when entering the detection circuitry taking account of the capacity of the input impedance. The higher this level the easier it will be to reject noise signals by means of a threshold circuit incorporated into the detection circuitry. For some commercially available detection circuits which have been especially developed for capacitive touch control switching systems a typical value to be achieved for the capacitance of the system of 3 pF may be cited. Lower capacitances may of course be used, depending on the sensitivity of the control circuitry, but as stated high values are preferred. As is well known, the capacitance (C) of a flat-plate capacitor is given by the formula C=K.(k a/d) where a is the total area, d is the thickness of the dielectric, k the dielectric constant and K is a constant depending on the units chosen. Thus for a given dielectric material, in order to increase the capacitance of a capacitor, one can make its plates of larger area or one can decrease the thickness of the dielectric sheet separating the plates.
Capacitive systems with which the present invention is concerned may be considered as two capacitors interconnected in series. One capacitor is formed by the second and first electrodes, and the other by the first and third electrodes. In fact there may be some capacitive interaction between the second and third electrodes, depending on the physical construction of the system, thus forming a third capacitor connected in parallel with the two series connected capacitors. The total capacitance of series interconnected capacitors is given by the reciprocal of the sum of the reciprocals of the capacitances of the individual capacitors, and from this it would seem to follow that for maximum capacitance in a capacitive system of the type with which the invention is concerned, the second and third electrodes should have equal area. Thus a typical system of the known type has a square first electrode on one face of a glass sheet and rectangular second and third electrodes on the other face of the sheet. In a practical example, the first electrode is formed by a tin oxide coating on a glass sheet 4.9 mm thick and is 26 mm square while the second and third electrodes are of a conductive silver-containing enamel deposited in rectangles measuring 26.times.12 mm and spaced apart by 2 mm. Such a system has a total capacitance of 3.8 pF.
The capacitance values where indicated in this application are obtained by measurement with a universal bridge WAYNE KERR, type B224. The measures are effected under normal ambient conditions. The capacitive switching system is disposed horizontally with the SnO.sub.2 coating located on top. Standard connecting wires are connected by means of terminal clamps fixed to wires 5 mm in length soldered to the second and third electrodes.
The electrodes of the capacitive switching system are built up from conductive materials applied to the dielectric sheet. Such conductive materials include conductive oxides or enamels as well as metals. The cost of the electrodes depends on the quantity of material used. For mass production of the switching system it is of course of interest to reduce the quantity of materials used. This is particularly true for instance when the electrodes are made starting from a silver containing paste. Any reduction of the quantity of paste used will favourably influence the cost price of the product. It is accordingly desirable to make switches with a small area and/or to reduce their active surface area.
There is another reason for preferring switches of small area, especially where a plurality of such switches are to be disposed in an array as, for example, for telephone dialling systems. In that case, an array of ten switches each occupying an area 26 mm square would take up too much space to be of practical value. It would be desirable to decrease the space taken up by the electrodes to an area approximately corresponding to that which would be occupied by a finger tip touching the switch, for example an area 12 mm square. However, a capacitive system consisting of a 12 mm square first electrode of tin oxide deposited on a surface of a sheet of glass 4.9 mm thick with second and third rectangular electrodes (also of tin oxide) each 12.times.5 mm and spaced apart by 2 mm has a capacitance of 1.4 pF. For the reasons explained before it is desirable to obtain a better level of capacitance for such an arrangement. This can be achieved by reducing the thickness of the dielectric. In the case of a glass sheet, by reducing the thickness to 1 mm, the total capacitance can be increased to 3.1 pF.
The fragility of a sheet of glass 1 mm thick renders it less than ideal for forming a surface which is to be repeatedly tapped.